This invention relates to the recovery of uranium from phosphate compounds and, more specifically, to the recovery of uranium from phosphoric acid produced by the acidulation of phosphate rock.
Most of the world's production of phosphate comes from marine phosphorites, and large deposits exist in Florida and the Western United States. These deposits generally contain from 50 to 200 ppm uranium (0.005 to 0.02%, or 0.1 to 0.4 pounds per ton). Although these concentrations are only 5% to 10% as high as those of commercially mined uranium ores, the vast extent of these deposits has made them of considerable interest as a uranium source for many years. It has been reported, for example, that mineable reserves of phosphate rock in the United States alone contain about 600,000 tons, or more than 1 billion pounds, of uranium.
During the early 1950's, considerable effort was directed toward methods of selectively leaching uranium from phosphate rock. However, it was found that alkaline leach methods were completely ineffective and that acid leaching required complete dissolution of the phosphate rock, consuming for example, several tons of sulfuric acid per pound of U.sub.3 O.sub.8 recovered. By contrast, uranium ores in the Western United States are primarily sandstone deposits containing 2-5 pounds of U.sub.3 O.sub.8 per ton of rock. There ores are essentially insoluble in acid and the uranium can be dissolved selectively by a relatively mild acid or alkaline digestion.
A large and increasing portion of commercial phosphate production is converted first to a relatively dilute phosphoric acid by the so-called "wet-process" (as distinguished from the furnace process which produces elemental phosphorus by direct reduction of the ore). The producer first manufactures sulfuric acid, then uses it to digest the rock. The chemical reaction forms phosphoric acid and calcium sulfate. The latter is filtered out, providing enormous quantities of gypsum, a waste product, and leaving an impure acid stream typically containing about 30% P.sub.2 O.sub.5. Most of the uranium in the original rock shows up in the 30% acid, and the various extraction processes have been developed to extract it therefrom. The 30% acid is generally evaporated to about 54% "merchant acid", which is either sold or used to manufacture a variety of products, chiefly fertilizers. The higher the acid concentration, the harder it is to extract the uranium, so the 30% stage is where the uranium extraction must take place. If uranium is not extracted, it ends up as a minor impurity in the various end products.
The 30% acid can be either "black" or "green". All of the phosphate rock contains measurable amounts of organic material such as humic acids. For example, Florida phosphate rock contains as much as 0.1% and more organic material, while Western phosphate rock contains substantially more. When phosphate rock containing solid organic material is acidulated with sulfuric acid, the aqueous phase takes up the solid organic material which is of such small particle size that much of it passes through the gypsum filter. This organic material is extremely fine and has a very slow filtration rate. Accordingly, it is not economically feasible to filter the acid to remove it.
The organic material, or black particulate, causes emulsions during uranium extraction. Richard H. Kennedy in an AEC report entitled "Recovery of Uranium from Low Grade Sandstone Ores and Phosphate Rock" presented to the International Atomic Energy Agency panel in June, 1966 points out the seriousness of the emulsion problem, and that it was never satisfactorily solved in all the uranium separation operations to that date. This problem was again acknowledged in an AEC report of the Oak Ridge National Laboratory in October 1970 (ORNL-4572), and these emulsions continue today to be a serious problem to those interested in uranium separation. In particular, these emulsions will collect at the organic-aqueous interface in any solvent extraction process that utilizes an organic extractant. The volume of emulsion generated is often such that flooding occurs and the equipment must be shut down and cleaned out to remove the emulsion.
If the original organic content of the rock is too high to be tolerated in phosphoric acid production, the rock is calcined before digestion to burn out the organic content, and the acid comes out with a greenish tint. All acid produced from rock originating in the Western United States must be calcined to remove organic materials before dissolution because of the high organic content. This green acid is easier to process, but is more costly to produce. Also, up to 30% of the uranium is lost to the gypsum when the calcined rock is digested. Mose central Florida rock, which has the highest uranium content, is processed to black acid.
The 30% acid leaving the filter is supersaturated with calcium sulfate in solution and additionally contains about 1 to 2% inorganic solids. If this acid is allowed to settle for several days, the inorganic solids along with the organic solids will settle to the bottom leaving a clear dark amber solution at the top. However, clarification by settling is not desirable in a phosphoric acid plant which typically produces 400 gallons per minute of phosphoric acid solution because of the vast storage facilities that would be required and the solids handling problems associated therewith. For example, in excess of sixty tons of inorganic solids are present in the filtered acid produced each day in a typical phosphoric acid plant. Since these solids will readily settle out, it is the practice in the industry to agitate all tanks in which the acid is stored to keep the solids suspended.
A number of prior processes have been developed to recover the minor amounts of uranium contained in wet-process phosphoric acid. In many of these processes, any hexavalent uranium is first reduced to the tetravalent state by the addition of iron and then extracted by contacting the acid with an organic extractant which has a high extraction coefficient for uranium in the tetravalent state. As is known, the coefficient of extraction (E.sub.a.sup.o) is a measure of the extraction power of a reagent and is defined as the ratio of the concentration of uranium in the organic phase to the concentration of uranium in the aqueous phase at equilibrium. Thereafter, the uranium is removed from the organic extractant by one of several methods. In one of these methods, the uranium is removed from the organic extractant by contacting the organic extractant with an aqueous solution containing hydrofluoric acid or some other reagent that reacts with uranium to form uranium precipitates.
In another of these methods, the uranium is removed from the organic extractant by oxidizing the uranium in the organic extractant to the hexavalent state. Thereafter, the uranium is stripped from the organic extractant with concentrated phosphoric or hydrochloric acid. Finally, the uranium is recovered from the stripping acid, for example, by reextraction with the same organic extractant. U.S. Pat. No. 2,859,092 to Bailes et al is illustrative of these prior art processes in which uranium is first oxidized to the hexavalent state.
Other prior art processes for recovering uranium employ organic extractants having a favorable extraction coefficient (E.sub.a.sup.o) for the extraction of hexavalent uranium from phosphoric acid. When one of these extractants is used, the uranium is stripped from the organic extractant by contacting the extractant with an aqueous solution containing compounds which react with hexavalent uranium to form uranyl compounds.
One way to remove the solid organic materials such as humic acids contained in wet-process phosphoric acid is described in commonly assigned U.S. Pat. No. 4,087,512 in which the acid is contacted with a liquid hydrocarbon such as kerosene so that the solid organic materials are suspended in an emulsion within the hydrocarbon phase. The emulsion can be removed either with the liquid hydrocarbon or withdrawn continuously from the interface between the acid and liquid hydrocarbon. Subsequent separation of the solid organic materials from the emulsion may be carried out by filtration, but the filtration rate is poor because of the mixed aqueous-organic nature of the emulsion which makes suitable wetting of the filter medium difficult.
During the manufacture of wet-process phosphoric acid, antifoaming agents are often used to reduce the foam produced during the reaction of the sulfuric acid and rock. Some of these defoamers and/or the organic materials present in the acid are soluble in the liquid hydrocarbon used to clean acid by the process described in U.S. Pat. No. 4,087,512. On continued contacting of this liquid hydrocarbon with the acid the organic soluble defoamers and/or other organic soluble materials accumulate in the liquid hydrocarbon. Liquid hydrocarbon losses with consequent fresh make-up normally brings about an equilibrium level of the defoamers and/or other organic soluble materials in the liquid hydrocarbon, but the concentration has been measured in practice to be higher than 10% W/V compared with the few hundred parts per million in the acid fed to the liquid hydrocarbon cleaning unit.
These defoamers and other organic soluble impurities in the acid have been found by Ralph E. Worthington and Donald A. Luke to be strongly detrimental to the extraction of uranium and other metals using an extractant such as a mixture of mono- and di-(alkylphenyl) esters of orthosphosphoric acid. Since some liquid hydrocarbon is always carried over from the cleaning system described above into the extraction stages, the defoamers and other organic soluble materials have been found by Worthington and Luke to accumulate in the organic extractant used to recover uranium and/or other metals from wet-process phosphoric acid and thereby to reduce the extraction coefficient of the extractant.
Accordingly, it is an object of the present invention to provide a process for removing solid organic materials from wet-process phosphoric acid.
A further object of the present invention is to provide a process for removing solid organic materials from wet-process phosphoric acid in a manner in which the solid organic materials can be readily separated by filtration or other suitable techniques.
Yet a further object of the present invention is to provide a process for removing organic soluble defoamers and other organic soluble materials from wet-process phosphoric acid.
A still further object of the present invention is to provide a process for removing solid organic materials, organic soluble defoamers and other organic soluble materials from wet-process phosphoric acid which is economical and minimizes the consumption of costly reagents.